This invention relates generally to ultra-high vacuum systems, and more particularly to in situ getter pumps used in ultra-high vacuum systems.
There are a number of processes which require ultra-high vacuum levels of, for example, 10.sup.-7 to 10.sup.-12 Torr. For example, high vacuum physics machines such as cyclotrons and linear accelerators often require a vacuum of the order of 10.sup.-8 -10.sup.-12 Torr. Also, in the semiconductor manufacturing industry, ultra-high vacuums of approximately 10.sup.-7 -10.sup.-9 Torr are often required in semiconductor processing equipment.
Several pumps are typically used in series or parallel to achieve ultra-high vacuum levels within a chamber. A mechanical (e.g. oil) pump is often used to reduce the pressure within a chamber to approximately 30-50 millitorr. These are often referred to as "high pressure" pumps since they only pump relatively high pressure gasses. Then, high or ultra-high vacuum pump systems, such as a molecular pump, ion pump, cryopump, turbo pump, etc. are used to reduce the pressure to approximately 10.sup.-7 -10.sup.-9 Torr. These are often referred to as "low pressure" pumps since they pump low pressure gasses. The pump-down time for a particular chamber can range from minutes to hours to days depending upon such factors as the size of the chamber, the capacity of the pumps, the conductance from the chamber to the pumps, and the desired final pressure.
In certain ultra-high vacuum applications, getter pumps have been used in conjunction with the aforementioned mechanical, molecular, and cryopumps. A getter pump includes getter materials (metal alloys) which have an affinity for certain non-noble gases. For example, depending upon the composition and temperature of the getter material, getter pumps have been designed which preferentially pump certain non-noble gases such as water vapor and hydrogen.
For example, getter pumps provided by SAES Getters, S.p.A. of Milan, Italy have been installed in particle accelerators for a number of years. The getter pump typically includes getter material encased in a stainless steel container. Getter pumps can operate from ambient temperatures to about 450.degree. C. depending upon on the species of gas to be pumped, the getter composition, etc. A preferred getter material for prior art SAES getter pumps is ST707.TM. getter material (which is an alloy of Zr-V-Fe) and which is produced by SAES Getters, S.p.A. of Milan, Italy. Another such material is ST101.TM. getter alloy, also available from SAES Getters, S.p.A., which is an alloy of Zr-Al. Some of these prior art getter pumps can be considered "in situ" pumps in that they are disposed within the high vacuum physics machines.
It has also been suggested that getter pumps be provided for semiconductor processing equipment. For example, in an article entitled "Non-Evaporable Getter Pumps for Semiconductor Processing Equipment" by Briesacher et al. some years back, it is suggested that any application which uses getters to purify processed gases for semiconductor processing can also utilize non-evaporable getter pumps for in situ purification and for the selective pumping of impurities.
The aforementioned Briesacher reference discloses that there are two possible operating scenarios for the use of getter pumps in a sputtering system. The first is the addition of the getter pump to the system to operate in parallel with conventional pumps (e.g. mechanical and cryopumps) of the system. In this scenario, the operation of the system is not modified in any way, and the getter pump merely serves as an auxiliary pump to lower the partial gas pressure of certain components of the residual gas in the chamber. The second scenario is to fill the chamber to a pressure in the range of 3.times.10.sup.-3 to 6.times.10.sup.-3 Torr, stopping the argon flow into the chamber, and sealing the chamber. The getter pump is then said to act as an "in situ" purifier for the argon. However, as discussed below, the pump is not truly "in situ" in that the active material is not within the volume of the processing chamber. An experimental processing chamber using such a getter pump was implemented at the department of electronics, Tohoku University, Japan under the guidance of Dr. Ohmi for some years.
The Briesacher reference discloses that a getter pump can be used in conjunction with a sputtering system, which is a type of semiconductor processing equipment. In one example of a typical sputtering system, a noble gas (usually argon) is pumped into a chamber and a plasma is created. The plasma accelerates argon ions towards the target causing material to become dislodged and to settle on the surface of the wafer. Getter pumps are well adapted for use with sputtering systems, since the only desired processing gas is a noble gas which is not pumped by the getter pump. Therefore, the getter pump can remove impurity gases from a sputtering chamber without affecting the flow of the noble gas required for the sputtering process.
The Briesacher reference was primarily an academic analysis of the practicality of using non-evaporable getter pumps in semiconductor processing equipment. Therefore, very little practical application of the theory is disclosed. Furthermore, while the Briesacher article uses the term "in situ" to describe scenario for the use of a getter pump, it is clear from the description that the getter pump is external to the chamber and is considered "in situ" only in that when the chamber is sealed and when no argon is flowing into the chamber, the volume within the getter pump can be considered to be connected to the chamber volume. However, it is not truly "in situ" in that the getter pump surfaces are within a volume that is connected to the chamber volume through a restrictive throat, which greatly limits the conductance between the chamber and the pump. For example, pumping through a throat of a pump may reduce conductance by 25% or more, and pumping through a throat of a pump having a heat shield (to shield the active members from the cryopump from heated members of the processing chamber) may reduce conductance 60% or more.
Sputtering systems used for the manufacture of integrated circuits have certain operational features which can be enhanced by in situ getter pumps in fashion that have not been addressed in the prior art. One such feature is the fact that production sputtering apparatus must operate at a number of different pressures and with different gas compositions. This feature, for example, is not present in particle accelerators, such as the aforementioned Princeton University particle accelerator, which are typically maintained at high vacuum. Nor was this feature addressed by the aforementioned Briesacher reference. More particularly, a sputter chamber of a commercial sputtering machine is often exposed to three entirely different environments. A first environment is present when the chamber is opened to the ambient atmosphere because of, for example, routine maintenance or for repair. Under such conditions the chamber is contaminated with atmospheric gases and pollutants. A second environment is present when the chamber is operated under ultra-high vacuum conditions, for example, less than 10.sup.-7 Torr, such as during loading and unloading of the chamber, and during pump-down to "base pressure" prior to processing. Finally, a third environment is present during processing, when the pressure of the argon gas in the sputtering chamber is at a pressure of a few millitorr.
In order to cycle between these various operating environments, a typical sputtering chamber is be coupled to a mechanical (high pressure) pump and a cryopump (low pressure pump). The mechanical pump will reduce the pressure in the chamber to approximately 30-50 millitorr and the cryopump (or other high vacuum pump, such as a turbo pump) will then be used to reduce the pressure in the chamber to approximately 10.sup.-7 -10.sup.-9 Torr.
It is commercially desirable to minimize the "transient" time between these various operating environments. For example, when going from atmospheric pressure to ultra-high vacuum conditions, it often takes 600-700 minutes for a traditional mechanical pump and cryopump to achieve the desired vacuum levels. Therefore, after every routine maintenance or repair, it can take ten hours or more for the sputter chamber to be ready to accept a wafer for processing. This can result in thousands or millions of dollars of "down-time" for the sputtering machine over its lifetime.
Since the total "pump down" time is more dependent upon the cryopump than the mechanical pump, one solution is to increase the size of the cryopump and the conductance to the pump. By "conductance" it is meant the ease with which a fluid (gas in this instance) flows from one volume (e.g. the processing chamber) to another volume (e.g. the pump chamber). Conductance is limited by the aperture size between the two chambers, which is typically the cross-sectional area of the throat of the cryopump, and the directness of the path between atoms, molecules, and particles to be pumped and the active surfaces within the cryopump. Unfortunately, increasing the size and conductance of the cryopump similarly increases the amount of argon that must flow into the process chamber to support the sputtering process. This has two undesirable side-effects. First, processing costs increase dramatically due to the high expense of argon gas. Second, the large amount of argon being pumped by the cryopump will quickly saturate the pump, requiring frequent "regenerations" (where trapped materials are released from the pump) and, therefore, more down-time for the system. In consequence, this solution of increasing the cryopump size is not commercially viable.
It is, in general, desirable to have a large capacity cryopump so that the period of time between regeneration cycles can be as long as possible. However, large cryopumps typically have large throats and large conductances. In the prior art, a baffle plate including, for example, one or more holes or other apertures can be placed over the mouth of the cryopump to reduce its conductance to acceptable levels. Alternatively, a smaller cryopump with a smaller conductance could be used without a baffle plate, or other restricting mechanisms can be used. However, with the smaller cryopump, the period of time between regeneration cycles would be less. Also, the base pressure with either of these solutions would be higher than with an unrestricted large cryopump. This is undesirable since the lower the base pressure, the cleaner the chamber.
Another possible solution to the problem of pumping a chamber of a sputter machine is to provide an additional cryopump, where one cryopump has a large conductance to pump-down the chamber to base pressures, and the other cryopump has a smaller conductance for pumping the chamber during processing. However, this solution also has its drawbacks. For one, cryopumps tend to take a fair amount of space since they require both liquid helium cryogenics and liquid nitrogen cryogenics to operate. Therefore, it is undesirable to add an additional cryopump in the often cramped space around semiconductor manufacturing equipment. Also, since cryopumps are quite expensive items, this would be an expensive solution. Furthermore, the smaller cryopump would have to be regenerated on a frequent basis. Also, each cryopump would require expensive and bulky gate valves and control systems. Finally, the chamber would likely have to be re-designed to accommodate two cryopumps.
Another possible solution would be to use a baffle plate having a variable size orifice. While this is theoretically appealing, such baffle plates for large cryopumps (e.g. cryopumps with 8" mouths) are not commercially available and are likely to be quite expensive and complicated to make. Furthermore, there may be some contamination problems associated with the mechanisms of a variable orifice.
Getter pumps have the interesting characteristic that they can preferentially pump certain gases. For example, by changing the composition of the material (typically a metal alloy), and its operating temperature, different gases are selectively pumped. For example, the aforementioned ST707 alloy preferentially pumps many non-noble gases at a temperature of about 350.degree. C., and preferentially pumps hydrogen gas at room temperatures (about 25.degree. C.). This characteristic of getter materials has been used to purify noble gases and nitrogen as disclosed in U.S. Pat. No. 5,238,469, issued Aug. 24, 1993 to Briesacher et al., assigned to SAES Pure Gas, Inc., which is incorporated herein by reference. However, the prior art does not disclose the use of an in situ getter pump which operates at several temperatures to preferentially pump several species of gases.